Using the serial analysis of gene expression (SAGE) method, we generated a gene expression profile of the rat hippocampus. A total of 76,790 SAGE tags was analyzed, allowing identification of 28,748 different tag species, each representing a unique mRNA transcript. The tags were divided into different abundancy classes, ranging from tags that were detected over 500 times to tags encountered only once in the 76,790 tags analyzed. The mRNA species detected more than 50 times represented 0.3% of the total number of unique tags while accounting for 22% of the total hippocampal mRNA mass. The majority of tags were encountered 5 times or less. The genes expressed at the highest levels were of mitochondrial origin, consistent with a high requirement for energy in neuronal tissue. At a lower level of expression, several neuron-specific transcripts were encountered, encoding various neurotransmitter receptors, transporters, and enzymes involved in neurotransmitter synthesis and turnover, ion channels and pumps, and synaptic components. Comparison of relative expression levels demonstrated that glutamate receptors are the most frequent neurotransmitter receptors expressed in the hippocampus, consistent with the important role of glutamatergic neurotransmission in the hippocampus, while GABA receptors were present at approximately 10-fold lower levels. Several kinases were present including CaMKII, which was expressed at high levels, consistent with its being the most abundant protein in the spines of hippocampal pyramidal cells. This is the first extensive inventory of gene expression in the hippocampus and serves as a reference for future studies aimed at elucidating hippocampal transcriptional responses under various conditions. Hippocampus 2001;11:430–444. © 2001 Wiley-Liss, Inc.